[PATCH] fix checkpoint header exports

[Sukadev Bhattiprolu]

Nathan Lynch [ntl at pobox.com] wrote:
| checkpoint_types.h is not useful to userspace.  Unexport it.

Hmm, I get this error when compiling ckpt-v16-dev when compiling for x86

home/suka/containers/build/linux-cr/usr/include/linux/fs.h:11: included
file 'linux/checkpoint_types.h' is not exported

(did not get this error on another machine).